1) Field of the Invention
This invention relates to a driving apparatus of a laser diode used as a light source of a laser printer and the like.
2) Related Arts
A conventional driving apparatus of a laser diode is disclosed in U.S. Pat. No. 4,625 315. A common laser diode unit is composed of a laser diode and a pin diode, such as the one shown in FIG. 1. In the figure, a laser diode LD and a pin diode PD, which is to monitor the intensity of light emitted by the laser diode LD, are built into a laser diode unit LDU. A cathode terminal TLC of the laser diode LD and an anode terminal TPA of the pin diode PD are drawn from the laser diode unit LDU. An anode terminal of the laser diode LD and a cathode terminal of the pin diode PD are connected to be made into a common terminal TCM, and drawn from the laser diode unit LCU. The common terminal TCM is generally impressed with positive voltage of about 5 volts (the reason the voltage is about 5 volts will be described later).
A driving apparatus 1 is composed of a constant current unit 11, which keeps the current constant, a modulation unit 12, which sends an alternating current IAC modulating depending on an input signal, and a detection unit 13, which outputs a feedback signal to stabilize the light emitted from the laser diode LD.
The constant current unit 11 and the modulation unit 12 are connected in parallel, their junction P1 and the cathode terminal TLC of the laser diode LD being directly connected.
The modulation unit 12 is provided with a buffer amplifier 32 whose output voltage modulates between positive and negative depending on an input signal, a resistor R1 to adjust the strength of the current being connected between an output terminal of the buffer amplifier 32 and the junction P1. The constant current unit 11 sends a direct current IDC having, for example, about half strength of the maximum laser current ILD, both the direct current IDC and the alternating current IAC of the modulation unit 12 being synthesized, and then sent to the laser diode LD.
The detection unit 13 amplifies voltage across a variable resistor 41 by the use of an amplifier 42 with an appropriate amplification gain, the voltage being generated by a current sent through the pin diode PD. The output voltage is inputted to a differential amplifier 31 as a feedback signal S2.
The above-mentioned driving apparatus, however, has a difficulty of improving responsibility of the pin diode PD. The higher a reverse bias voltage impressed on the pin diode is, the more improved the responsibility is. However, the conventional driving apparatus demands restricting voltage at the cathode terminal TLC as low as several volts depending on the characteristics of the constant current unit 11 and the modulation unit 12. In addition, voltage between both ends of the laser diode LD is almost constant. As a result, the common terminal TCM can only be impressed as a low voltage of 5 volts.